a(n) = (2^n + 1)^2.

Terms

a(0) =4a(1) =9a(2) =25a(3) =81a(4) =289a(5) =1089a(6) =4225a(7) =16641a(8) =66049a(9) =263169a(10) =1050625a(11) =4198401a(12) =16785409a(13) =67125249a(14) =268468225a(15) =1073807361a(16) =4295098369a(17) =17180131329a(18) =68720001025a(19) =274878955521